Acouple of years ago, I saw a small field of patch in a sea of genetically modified corn and soybeans. It was an unusual sight. I asked my cousins, who still farm what my dad always called the “home place,” whether someone had added oats to the rotation of crops being planted. The answer was no.

The purpose of that patch of oats was manure mitigation. The waste that had been sprayed on that field came from a hog confinement operation, and oats were the only crop that would put such concentrated, nearly toxic manure to nutritional use and do it quickly.

Oats used to be a common sight all over the Midwest. They were often sown with alfalfa as a “nurse crop” to provide some cover for alfalfa seedlings back when alfalfa was also a common sight. Until about 30 years ago, you could find all sorts of crops growing on Iowa farms, and livestock. Since then two things have happened. All the animals have moved indoors, into crowded confinement operations. And the number of crops has dwindled to exactly two: corn and soybeans.

My uncle Everon, who died last summer, farmed the home place when I was growing up. He would have been surprised to learn that he was following the principles of an early 18th-century agricultural experimenter named Charles Townshend, who, apart from his fascination with turnips, was every inch a viscount. Townshend’s discovery — borrowed from Dutch and Flemish farmers — was that crops grow better, with fewer weeds and pest problems, if they are rotated in a careful sequence.

Townshend’s rotation — like the ones George Washington and Thomas Jefferson used — included clover, wheat, other small grains and turnips, which made good winter food for sheep and cattle. My uncle grew no turnips, but he, like all his neighbors, was using his own version of the four-crop system, at the heart of which was alfalfa.

Getting to the four-crop rotation wasn’t easy, historically speaking. The Romans knew about crop rotation, but by the Middle Ages, farming was based on the practice of letting the land lie fallow, unplanted — resting it, in other words. The purpose of that practice, like crop rotation itself, is to prevent the soil from becoming exhausted when the same crop is sown over and over again. In early American agriculture, only sophisticated farmers like Washington and Jefferson were using crop rotations in their fields. There was simply too much good land available. It was too easy to farm a piece and then move on when the soil was depleted.

In one sense, that is still how modern agriculture works. You look to the future and discard the past. A modern rotation includes only corn, soybeans, fertilizer and pesticides. Whatever you may think about genetically modified crops, the switch to those varieties has driven the rush to the two-crop system. Those crops are designed to tolerate the presence of herbicides. The result is that farmland has been inundated with glyphosate, the herbicide genetically modified crops are engineered for.

The very structure of the agricultural system, as it stands now, is designed to return the greatest profit possible, not to the farmers but to the producers of the chemicals they use and the seeds they plant. And because those chemicals depend on fossil energy, the entire system is inherently unsustainable. What farmers used to return to the soil in the form of labor and animal manure — not the toxic kind you find in livestock confinement systems — they now must purchase, just the way they buy diesel for their tractors.

In fact, as a recent study by agronomists from the U.S. Agriculture Department, Iowa State University and the University of Minnesota shows, there’s nothing obsolete about four-crop rotation. It produces the same yields, it sharply reduces the toxicity of freshwater runoff, and it eliminates many of the problems associated with genetically modified crops, including the emergence of glyphosate-resistant weeds. It’s also simply better for the soil. A four-crop rotation using conventional crop varieties, along with much lower applications of fertilizer and herbicides and some animal manure, works every bit as well as the prevailing monotony of corn and soybeans.

This study is a reminder of something essential. Modern agriculture is driven by diminishing biological diversity and relentless consolidation, from the farms themselves to the processors and the distributors of the crops and livestock. But you cannot consolidate the soil. It is a complex organism, and it always responds productively to diversity. The way we farm now undervalues and undermines good soil. Our idea of agricultural productivity and efficiency must include the ecological benefits of healthy soil. The surest way to improve the soil is to remember what industrial agriculture has chosen to forget.